Circulatory System and Blood Vessels

Chapter 21 Blood Vessels and Circulation

Arteries of the Body

  • Arteries of the Upper Limb- Subclavian, Axillary, Brachial, Radial, Ulnar

  • Arteries of the Head and Trunk- Internal Carotid, External Carotid, Left Common Carotid, Brachiocephalic, Aortic Arch, Pulmonary Trunk, Left Coronary, Right Coronary, Thoracic Aorta, Celiac Trunk, Splenic, Renal, Superior Mesenteric, Abdominal Aorta, Inferior Mesenteric, Common Iliac, Internal Iliac

  • Arteries of the Lower Limb- Deep Femoral, Femoral, Popliteal, Anterior Tibial, Posterior Tibial, Fibular, Dorsalis Pedis

Key Concepts

Function of the Circulatory System

  • Carries blood, exchanges nutrients/waste/gases, transports hormones/immune components/enzymes.

  • Regulates blood pressure, flow, and volume.

Structure and Types of Blood Vessels

  • Continuous passageway: Heart \rightarrow Arteries \rightarrow Arterioles \rightarrow Capillaries \rightarrow Venules \rightarrow Veins \rightarrow Heart.

  • Types:

    • Arteries: Elastic, muscular, arterioles, metarterioles

    • Capillaries: Continuous, fenestrated, sinusoid

    • Veins: Medium-sized, small-sized, venules

  • Types of Circulation: pulmonary and systemic.

Histology of Blood Vessels

Structural Features of Blood Vessels

  • Tunica Externa (Adventitia): Outermost, dense irregular connective tissue; anchors and protects. Contains vasa vasorum and nervi vasorum.

  • Tunica Media: Thick smooth muscle layer; regulates diameter, blood flow, and pressure (vasoconstriction/vasodilation). Thicker in arteries.

  • Tunica Intima (Interna): Innermost, simple squamous epithelium (endothelium); smooth surface, minimizes friction, regulates clotting.

Types of Arteries

  • Elastic Arteries: Large (aorta, pulmonary trunk), more elastic tissue. \rightarrow Withstand pressure, propel blood continuously.

  • Muscular Arteries: Medium-sized, thick tunica media with smooth muscle. \rightarrow Regulate blood flow to organs via vasodilation/vasoconstriction (e.g., Brachiocephalic, subclavian).

  • Arterioles: Smallest arteries, fewer smooth muscle layers. \rightarrow Regulate peripheral resistance and capillary blood flow.

Capillaries

  • Smallest diameter (4 to 10 microns).

  • Endothelium, basement membrane, loose connective tissue. \rightarrow Facilitates rapid diffusion/filtration due to thin walls and large surface area.

  • Found in high metabolic tissues. Involved in thermoregulation.

Types of Capillaries

  • Continuous Capillaries: No gaps, less permeable. Found in muscles, nervous tissue, skin.

  • Fenestrated Capillaries: Endothelial cells with pores (fenestrae), high permeability. Found in intestinal villi, kidneys.

  • Sinusoidal Capillaries: Large diameter, large fenestrae, incomplete basement membrane, large gaps. \rightarrow Allows passage of large molecules and cells. Found in liver, spleen, bone marrow.

Capillary Networks

  • Branching networks regulated by precapillary sphincters.

  • Blood flow: Arterioles \rightarrow Metarterioles \rightarrow Capillary Networks. Metarterioles can bypass capillaries via thoroughfare channels.

Anastomoses

  • Vascular Anastomoses: Intersecting blood supply from two or more vessels. \rightarrow Redundant pathways, preventing ischemia.

  • Arterial Anastomoses: Connections between arteries.

  • Arteriovenous Anastomoses: Direct flow from arterioles to venules, bypassing capillaries.

  • Venous Anastomoses: Connect veins, offer alternative routes; less critical blockages than arterial.

Veins

  • Venules: Drain capillary networks; few smooth muscle cells. Primary sites for leukocyte diapedesis.

  • Small Veins: Continuous smooth muscle and tunica adventitia; collect blood from venules.

  • Large Veins: Thinner walls than arteries; predominant tunica externa. "Capacitance vessels" \rightarrow hold large blood volume.

Portal Veins

  • Connect two capillary networks without passing through the heart first.

  • Three portal systems in humans:

    • Hepatic Portal System: GI capillaries \rightarrow liver.

    • Hypothalamo-hypophyseal Portal System: Hypothalamus \rightarrow pituitary gland (hormone transport).

    • Renal Nephron Portal System: Within kidney structures (urine formation).

Valves in Veins

  • Present in veins > 2 \text{ mm} diameter (especially in limbs) \rightarrow prevent backflow against gravity. More valves in lower extremities.

  • Varicose veins: Dilated, painful veins due to malfunctioning valves \rightarrow blood pooling, phlebitis.

Divisions of the Circulatory System

Pulmonary Circulation

  • Heart \leftrightarrow Lungs: Deoxygenated blood from right ventricle to lungs; oxygenated blood returns to left atrium.

Systemic Circulation

  • Heart \leftrightarrow Body: Oxygenated blood from left ventricle via aorta to body; deoxygenated blood returns to right atrium.

Pathways of Blood Flow

Systemic Arteries

  • Ascending Aorta: Right and left coronary arteries.

  • Aortic Arch: Branches into Brachiocephalic, Left common carotid, Left subclavian.

  • Descending Aorta: Thoracic aorta \rightarrow Abdominal aorta \rightarrow Common iliac arteries.

Major Arteries of the Head and Neck

  1. Brachiocephalic Artery: Branches into right common carotid and right subclavian.

  2. Left Common Carotid Artery: Supplies left neck/head.

  3. Left Subclavian Artery: Supplies left upper limb.

  4. Common carotid arteries: Branch into internal (brain) and external (face/neck).

  5. Carotid sinus: Baroreceptors (pressure) at common carotid branch; regulates heart rate/vessel tone. Carotid body: chemoreceptors (blood \text{O}_2, \text{CO}_2, \text{pH}) adjacent.

Arteries Supplying the Brain

  1. Vertebral Arteries: From subclavian; form basilar artery \rightarrow supplies pons, midbrain, cerebellum.

  2. Internal Carotid Arteries: Contribute to Circle of Willis (arterial anastomosis \rightarrow alternative brain blood routes).

Major Branches of the Aorta

  1. From the aortic arch: Brachiocephalic, Left common carotid, Left subclavian.

  2. Thoracic Aorta: Visceral (esophagus, bronchi), parietal (thoracic walls).

  3. Abdominal Aorta: Unpaired (celiac trunk, superior/inferior mesenteric \rightarrow GI tract) and paired branches (renal, gonadal, common iliac \rightarrow lower limbs/pelvis).

Circulatory Dynamics and Regulation

Blood Flow Dynamics

  • Influenced by viscosity, resistance, pressure gradients.

  • Blood flow (F) \propto \Delta P / R (F = \frac{\Delta P}{R}).

  • Resistance (R): Determined by vessel length, diameter, and blood viscosity.

Blood Pressure Control

  • Force exerted by blood against vessel walls; controlled via systemic vascular resistance and cardiac output.

  • Measured with sphygmomanometer; Korotkoff sounds.

  • Phases: Systolic (peak during ventricular contraction); Diastolic (minimum during ventricular relaxation).

  • Classifications: Normal: Systolic <120 \text{ mmHg} and Diastolic <80 \text{ mmHg} .

Factors Influencing Blood Pressure

  • Blood pressure decreases from heart due to cumulative resistance.

  • Blood flow: Fastest in aorta, slowest in capillaries (large total cross-sectional area) to optimize exchange.

Capillary Exchange Processes

  • Main method: diffusion. Also filtration and reabsorption.

  • Regulated by Starling forces (hydrostatic and osmotic pressures) \rightarrow fluid out (filtration) at arterial end, in (reabsorption) at venous end.

Venous Return Mechanisms

  • Muscular Pump: Skeletal muscle contractions compress veins, push blood to heart.

  • Respiratory Pump: Pressure changes during breathing aid blood flow to heart.

  • Sympathetic Venoconstriction: Sympathetic nervous system constricts smooth muscle in veins; increases venous return.

Long-Term Blood Pressure Regulation

  • Hormonal mechanisms (ADH, aldosterone, ANP) regulate blood pressure via changes in blood volume and electrolyte balance.

Key